1. Field of the Invention
This invention is directed to a method. More specifically, this invention concerns itself with extending the useful life of polymers by enhancement of their resistance to degradation by ultraviolet light.
2. Description of The Prior Art
Polymers have in the past and continue to provide an attractive substitute for the more traditional types of structural materials (e.g., wood and metals) because of relatively inexpensive materials and fabrication costs. As polymers continue to find new applications in, for example, the fabrication of automotive parts and building materials, they must also become more durable and capable of withstanding prolonged exposure to a variety of degradative forces. Degradation of polymers can be caused by exposure to light, heat, moisture and/or air. Such degradation is usually manifest by either a partial or total loss of structural integrity, changes in light transmission properties, changes in color, loss or reduction in flexibility and/or resiliency, or any combination of the above phenomenon. Those attempting to avoid polymer degradation have generally selected from among three possible approaches: (a) elimination or reduction of the degradative forces; (b) isolation of the sensitive polymer material from the degradative forces; or (c) modification of the polymer composition to enhance its resistance to degradative forces. The latter approach is generally preferable since it does not require elaborate engineering or structural changes in the polymer product environment.
There are a variety of additives which have been disclosed as suitable for enhancing the resistance of polymers to one or more of the degradative forces discussed hereinabove. These additives (hereinafter referred to as "stabilizers") can be physically combined with or engrafted upon the environmentally sensitive polymer, thereby prolonging its useful life in the hostile degradative environment. Stabilizers are available which can enhance the polymers resistance to more than one of the degradative forces and conversely, a stabilizer which is effective for prevention of, for example, oxidative degradation may have little if any effect upon the polymers resistance to other degradative agents. Thus, it is not uncommon for polymers to contain a variety of stabilizer materials, each being present for the prevention of a particular degradative reaction.
One of the more difficult to control of the degradative forces is irradiation of the polymer by ultraviolet light. The impact of such irradiation will of course vary depending upon the intensity and duration of exposure and thus may manifest itself only after a prolonged interval. The irradiation of polymers with ultraviolet light can often times cause cross-linking of these materials thereby reducing its resiliency and/or impact resistance. Changes in color and opacity are also often effected by prolonged exposure of the polymer to ultraviolet light. While many materials are known, and commercially available, as stabilizers against ultraviolet light degradation, the degree of protection afforded by such agents is often concentration dependent. The following list of references are illustrative of the various types of ultraviolet absorbers commonly used in conjunction with polymeric materials sensitive to UV photodearadation: U.S. Pat. Nos. 3,362,929; 3,362,930; 3,829,292; 3,901,849; 3,910,918; 3,939,164; published patent application numbers b402,162 and b571,638; U.K. Pat. No. 999,806; and an article appearing in J. Am. Chem. Soc., Vol. 60: 1458 et seq. (1938). In addition, commonly assigned U.S. patent applications Ser. No. 697,345 and 697,387 (both filed on June 18, 1976) disclose compounds which are highly effective as ultraviolet light stabilizers for polymeric materials. The compounds disclosed in the above-referenced pending patent applications are substituted decahydroquinolines and are highly effective in enhancing the resistance of polyolefins to photodegradation by ultraviolet light.
The stabilization of polymeric materials with other heterocyclic compounds, such as oxazolines (also commonly known as "dihydro oxazoles") and oxazolidines (also commonly known as "tetrahydro oxazoles"), has also been previously reported in the open literature. For example, U.S. Pat. No. 3,511,789 discloses the use of oxazolines as foam stabilizers; U.S. Pat. No. 3,703,496 discloses the use of oxazolines as inhibiting the hydrolytic cleavage of phosphite esters and compositions containing phosphite esters and epoxy resins; U.S. Pat. No. 3,380,975 discloses the thermal stabilization of PVC by copolymerization of a vinyl oxazoline monomer and vinyl chloride monomer; and U.S. Pat. No. 3,594,387 discloses the use of 2-imino-oxazolidines as inhibiting the hydrolytic cleavage of simple esters and polyesters. Oxazolines have also been reported in U.S. Pat. No. 3,547,863 as anti-static agents for a variety of polyolefins (e.g., ethylene, propylene, 4-methyl-1-pentene, styrene, isoprene and a variety of their respective copolymers), and in U.S. Pat. No. 3,810,826 as accelerators in the radiation curing (accelerated particulate radiation) of certain polyesters and copolymers of vinyl esters and non-volatile vinyl monomers (e.g., alkenyl aromatics, vinyl carboxylic acids, vinyl nitriles, alkyl and hydroxyalkyl esters of vinyl carboxylic acids, vinyl amides, and mixtures of the foregoing).
In summary, the stabilization of polymers against changes in their physical, chemical and/or electrical properties can be readily achieved by the incorporation of one or more stabilizer compounds within the environmentally unstable host polymer matrix. A variety of such stabilizer agents have been previously disclosed which are specific for the retardation of only one such degradative reaction. As noted herein, oxazolines and oxazolidines are among the materials reported in the literature which are effective for the stabilization of polymers against a number of such degradative forces. However, the disclosure of certain substituted oxazolines as accelerators in a process for radiation curing of certain polymers would tend to suggest that the presence of these substances and structurally similar compounds in polymers can have an unsettling effect upon the host polymer matrix (e.g., the promotion of cross-linking of the host polymer upon the intense exposure of the polymer to electro-magnetic radiation within a certain band width). Thus, the literature discussed hereinabove would lead one to conclude that oxazolines and structurally similar compounds would be undesirable for the stabilization of ultraviolet light sensitive polymers because of such compound's apparent ability to promote radiation initiated curing (cross-linking) of certain polymeric resins.